HomeMy WebLinkAbout1.20 Subsoil Study for Foundation Design5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED SUNLIGHT PARKWAY
NEAR 2808 COUNTY ROAD 117
GARFIELD COUNTY, COLORADO
PROJECT NO. 23-7-413
SEPTEMBER 13, 2023
PREPARED FOR:
DM NEUMAN CONSTRUCTION
ATTN: JASON NEUMAN
310 19TH STREET
GLENWOOD SPRINGS, COLORADO 81601
jmn@dmneuman.com
Kumar & Associates, Inc. ® Project No. 23-7-413
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ....................................................................................... - 1 -
PROPOSED CONSTRUCTION ................................................................................................ - 1 -
SITE CONDITIONS ................................................................................................................... - 1 -
GEOLOGY ................................................................................................................................. - 1 -
FIELD EXPLORATION ............................................................................................................ - 3 -
SUBSURFACE CONDITIONS ................................................................................................. - 3 -
FOUNDATION BEARING CONDITIONS .............................................................................. - 4 -
DESIGN RECOMMENDATIONS ............................................................................................ - 4 -
FOUNDATIONS .................................................................................................................... - 4 -
FOUNDATION AND RETAINING WALLS ....................................................................... - 5 -
FLOOR SLABS ...................................................................................................................... - 6 -
UNDERDRAIN SYSTEM ..................................................................................................... - 6 -
SURFACE DRAINAGE ......................................................................................................... - 6 -
LIMITATIONS ........................................................................................................................... - 7 -
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURES 3 TO 5 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 6 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. ® Project No. 23-7-413
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed six lot subdivision to be located
near 2808 Four Mile Road, Garfield County, Colorado. The project site is shown on Figure 1.
The purpose of the study was to develop recommendations for the foundation design. The study
was conducted in accordance with our agreement for geotechnical engineering services to DM
Neuman Construction dated June 30, 2023.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils obtained during the field
exploration were tested in the laboratory to determine their classification, compressibility or
swell and other engineering characteristics. The results of the field exploration and laboratory
testing were analyzed to develop recommendations for foundation types, depths and allowable
pressures for the proposed building foundation. This report summarizes the data obtained during
this study and presents our conclusions, design recommendations and other geotechnical
engineering considerations based on the proposed construction and the subsurface conditions
encountered.
PROPOSED CONSTRUCTION
The proposed residences will be two story, wood-frame structures over crawlspace or basement
levels. Ground floors will be over crawlspace or slab-on-grade. Grading for the structures is
assumed to require cut depths between about 3 to 10 feet. We assume relatively light foundation
loadings, typical of the proposed type of construction.
If building loadings, location or grading plans change significantly from those described above,
we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The site is undeveloped and vegetated with scattered scrub oak and juniper trees with an
understory of grass and weeds. The proposed development area is in a narrow valley that is
gently sloping down to the north. The east side of the valley slopes steeply down to the west.
The west side of the valley slopes steeply up to Four Mile Road located just west of the site
GEOLOGY
Regional geologic mapping shows that project site is apparently underlain by the Maroon
Formation. Surficial soil deposits at the site consist of Holocene- to late Pleistocene-age
intermediate debris flow deposits (Kirkham and Others, 2014) overlying the Maroon Formation.
The Garfield County Geologic Hazard Maps identify the surficial deposits as colluvium
including potential hydro-compressive soils.
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Kumar & Associates, Inc. ® Project No. 23-7-413
Geologic Site Assessment: The project site geology should not present major constraints or
unusually high risks to the proposed development. There are, however, several conditions of a
geologic nature that should be considered. Geologic conditions that should be considered, their
potential risks, and mitigations to reduce the potential risks are discussed below. The site could
experience moderate levels of earthquake related ground shaking. No rock outcrops capable of
producing rockfall that would impact the proposed building areas were observed.
Potential Debris Flow – There is one small drainage channel on the steep slope to the east of the
proposed development area. There is not a significant basin associated with this channel and
channelized flow should not be expected at the proposed development area. Minor sheet flow
off of the steep slope to the east of the proposed development area should be expected, but
should not present a hazard with proper drainage and grading for the development. Surface
drainage recommendations are presented below.
Potential Flooding - According to the “Flood Insurance Rate Map”, map number 082051445B by
the Federal Emergency Management Agency (FEMA, 1986); the site is located in Zone C
(unshaded – areas of minimal flood hazard).
Pre-Existing Man-Placed Fill – There is an undetermined depth of previously placed fill material
across a majority of the subject site. We are not aware of any records documenting the
placement or compaction of the previously placed fill. It is unlikely that the pre-existing fill will
be suitable for support of building foundations.
Seismicity - Historic earthquakes within 150 miles of the project site have typically been
moderately strong with magnitudes less than 5.5 and maximum Modified Mercalli Intensities
less than VI, (Widmann and Others, 1998). The largest historic earthquake in the project region
occurred in 1882. It was located in the northern Front Range and had an estimated magnitude of
about M6.4 ± 0.2 and a maximum intensity of VII. Historic ground shaking at the project site
associated with the 1882 earthquake and the other larger historic earthquakes in the region does
not appear to have exceeded Modified Mercalli Intensity VI (Kirkham and Rogers, 2000).
Modified Mercalli Intensity VI ground shaking should be expected during a reasonable exposure
time for the residences, but the probability of stronger ground shaking is low. Intensity VI
ground shaking is felt by most people and causes general alarm, but results in negligible damage
to structures of good design and construction.
Using estimated shear wave velocities for the subsoils at the site based on our experience in the
area and sample blow count drives from the exploratory boring drilled the site for the
preliminary subsoil study, the seismic soil profile at the project site should be considered as
Class C, very dense soil and soft rock, as described in the 2018 International Building Code,
unless site specific shear wave velocity studies show otherwise. Based on the subsurface profile
and the anticipated ground conditions, liquefaction is not a design consideration. Using the
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Kumar & Associates, Inc. ® Project No. 23-7-413
USGS National Earthquake Hazard Reduction Program online database, the following
probabilistic ground motion values are reported for the project site.
Intensity Measure Type Intensity Measure Level
2 percent in 50 Years
0.2 Sec. Spectral Acceleration Ss 0.463
1.0 Sec. Spectral Acceleration S1 0.090
The USGS National Earthquake Hazard Reduction Program online database also indicates a peak
ground acceleration (PGA) of 0.304g at the subject site. The PGA is the lower of either the
deterministic or probabilistic value with a 2% exceedance probability for a 50-year exposure
time at the project site (statistical recurrence interval of 2,500 years).
FIELD EXPLORATION
The field exploration for the project was conducted on July 18, 2023. Five exploratory borings
were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions. The
borings were advanced with 4-inch diameter continuous flight augers powered by truck-mounted
CME-45B drill rig. The borings were logged by a representative of Kumar & Associates, Inc.
Samples of the subsoils were taken with 1⅜ inch and 2 inch I.D. spoon samplers. The samplers
were driven into the subsoils at various depths with blows from a 140 pound hammer falling 30
inches. This test is similar to the standard penetration test described by ASTM Method D-1586.
The penetration resistance values are an indication of the relative density or consistency of the
subsoils. Depths at which the samples were taken and the penetration resistance values are
shown on the Logs of Exploratory Borings, Figure 2. The samples were returned to our
laboratory for review by the project engineer and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils, below ½ foot of topsoil, generally consist of about 6 to 14 feet of medium stiff to stiff,
sandy silty clay overlying relatively dense, silty sandy gravel with cobble and boulders. Borings
3 and 5, located on the west side of the site, encountered 9 to 15 feet of clayey sand to sandy clay
fill overlying the natural sand and gravel soils. Drilling in the dense granular soils with auger
equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in
the deposit.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and gradation analyses. Results of swell-consolidation testing performed on relatively
undisturbed drive samples, presented on Figures 3 to 5, indicate low to moderate compressibility
under conditions of loading and wetting. A shallow sample form Boring 1 showed a low
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Kumar & Associates, Inc. ® Project No. 23-7-413
expansion potential and a shallow sample from Boring 4 showed a minor collapse (settlement
under constant load) when wetted under light loading. Results of a gradation analysis performed
on a small diameter drive sample from Boring 1 (minus 1½-inch fraction) of the coarse granular
subsoils are shown on Figure 6. The laboratory testing is summarized in Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist to moist.
FOUNDATION BEARING CONDITIONS
The natural clay soils encountered at the site should be suitable for support of spread footing
foundations. The clay fill soils encountered in Borings 3 and 5, on the west side of the site,
appear to have variable density and consistency and settlement/heave potential. The clay fill soils
should be further evaluated prior to construction of Lots 1 to 5. It may be possible to place a
depth of structural fill below footings to mitigate potential settlement. Placing the houses on
basements in this area would help to reduce the potential risk of settlement.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the residences be founded with spread footings
bearing on the natural granular soils. The area of Lots 1 to 6 should be further evaluated to
determine the depth and extent of the on-site fill.
The design and construction criteria presented below should be observed for a spread footing
foundation system placed on the natural clay soils.
1) Footings placed on the undisturbed natural clay soils should be designed for an
allowable bearing pressure of 1,500 psf. Based on experience, we expect
settlement of footings designed and constructed as discussed in this section will
be about 1 inch or less.
2) The footings should have a minimum width of 18 inches for continuous walls and
2 feet for isolated pads.
3) Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
of foundations at least 36 inches below exterior grade is typically used in this
area.
4) Continuous foundation walls should be reinforced top and bottom to span local
anomalies such as by assuming an unsupported length of at least 12 feet.
Foundation walls acting as retaining structures should also be designed to resist
lateral earth pressures as discussed in the "Foundation and Retaining Walls"
section of this report.
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Kumar & Associates, Inc. ® Project No. 23-7-413
5) All existing loose fill, topsoil and any loose or disturbed soils should be removed
and the footing bearing level extended down to the relatively stiff, natural clay
soils. The exposed soils in footing area should then be moistened and compacted.
If water seepage is encountered, the footing areas should be dewatered before
concrete placement.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structures which are laterally supported and can be expected to
undergo only a slight amount of deflection should be designed for a lateral earth pressure
computed on the basis of an equivalent fluid unit weight of at least 55 pcf for backfill consisting
of the on-site fine-grained soils and at least 45 pcf for backfill consisting of imported granular
materials. Cantilevered retaining structures (site walls) which are separate from the residences
and can be expected to deflect sufficiently to mobilize the full active earth pressure condition
should be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit
weight of at least 45 pcf for backfill consisting of the on-site fine-grained soils and at least 35 pcf
for backfill consisting of imported granular materials.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The
pressures recommended above assume drained conditions behind the walls and a horizontal
backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will
increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain
should be provided to prevent hydrostatic pressure buildup behind walls.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at a moisture content near optimum. Backfill in pavement and
walkway areas should be compacted to at least 95% of the maximum standard Proctor density.
Care should be taken not to overcompact the backfill or use large equipment near the wall, since
this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
backfill should be expected, even if the material is placed correctly, and could result in distress to
facilities constructed on the backfill. Backfill should not contain organics, debris or rock larger
than about 6 inches.
The lateral resistance of foundation or retaining wall footings will be a combination of the
sliding resistance of the footing on the foundation materials and passive earth pressure against
the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated
based on a coefficient of friction of 0.30. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 325 pcf. The
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Kumar & Associates, Inc. ® Project No. 23-7-413
coefficient of friction and passive pressure values recommended above assume ultimate soil
strength. Suitable factors of safety should be included in the design to limit the strain which will
occur at the ultimate strength, particularly in the case of passive resistance. Fill placed against
the sides of the footings to resist lateral loads should be compacted to at least 95% of the
maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade
construction. To reduce the effects of some differential movement, floor slabs should be
separated from all bearing walls and columns with expansion joints which allow unrestrained
vertical movement. Floor slab control joints should be used to reduce damage due to shrinkage
cracking. The requirements for joint spacing and slab reinforcement should be established by the
designer based on experience and the intended slab use. A minimum 4 inch layer of free-
draining gravel should be placed beneath basement level slabs to facilitate drainage. This
material should consist of minus 2-inch aggregate with at least 50% retained on the No. 4 sieve
and less than 2% passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95% of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the
on-site soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
mountainous areas that local perched groundwater can develop during times of heavy
precipitation or seasonal runoff. Frozen ground during spring runoff can create a perched
condition. We recommend below-grade construction, such as retaining walls, crawlspace and
basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain
system.
The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above
the invert level with free-draining granular material. The drain should be placed at each level of
excavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimum 1% to
a suitable gravity outlet. Free-draining granular material used in the underdrain system should
contain less than 2% passing the No. 200 sieve, less than 50% passing the No. 4 sieve and have a
maximum size of 2 inches. The drain gravel backfill should be at least 1½ feet deep.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the residences have been completed:
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Kumar & Associates, Inc. ® Project No. 23-7-413
1) Inundation of the foundation excavations and underslab areas should be avoided
during construction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95% of the maximum standard Proctor density in pavement and slab areas
and to at least 90% of the maximum standard Proctor density in landscape areas.
3) The ground surface surrounding the exterior of the building should be sloped to
drain away from the foundation in all directions. We recommend a minimum
slope of 12 inches in the first 10 feet in unpaved areas and a minimum slope of
3 inches in the first 10 feet in paved areas. Free-draining wall backfill should be
covered with filter fabric and capped with about 2 feet of the on-site soils to
reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area at this time. We make no warranty either express or implied.
The conclusions and recommendations submitted in this report are based upon the data obtained
from the exploratory borings drilled excavated at the locations indicated on Figure 1, the
proposed type of construction and our experience in the area. Our services do not include
determining the presence, prevention or possibility of mold or other biological contaminants
(MOBC) developing in the future. If the client is concerned about MOBC, then a professional in
this special field of practice should be consulted. Our findings include interpolation and
extrapolation of the subsurface conditions identified at the exploratory borings and variations in
the subsurface conditions may not become evident until excavation is performed. If conditions
encountered during construction appear different from those described in this report, we should
be notified so that re-evaluation of the recommendations may be made.
This report has been prepared for the exclusive use by our client for design purposes. We are not
responsible for technical interpretations by others of our information. As the project evolves, we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to verify that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. We recommend on-site observation
of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 23-7-413
SAMPLE LOCATION NATURAL
MOISTURE CONTENT
NATURAL
DRY DENSITY
GRADATION
PERCENT PASSING NO. 200 SIEVE
ATTERBERG LIMITS UNCONFINED
COMPRESSIVE STRENGTH SOIL TYPE BORING DEPTH GRAVEL SAND LIQUID LIMIT PLASTIC INDEX (%) (%)
(ft) (%) (pcf) (%) (%) (psf)
1 2 10.8 108 Sandy Silty Clay
9 6.0 51 31 18 Silty Sandy Gravel
2 4 15.2 105 57 1,600 Sandy Silty Clay
3 2 8.8 116 33 Silty Clayey Sand with
Gravel (Fill)
9 16.6 108 66 Sandy Silty Clay (Fill)
4 2 8.9 89 Sandy Silty Clay
9 8.3 92 72 Sandy Silty Clay
5 4 7.8 101 35 Silty Clayey Sand with
Gravel (Fill)
9 13.7 103 Sandy Silty Clay
14 13.9 107 68 Sandy Silty Clay